It's a nice experience, playing on one.
However, you don't need them for SC or Tekken. My TE stick is in my closet.
With a stick, you can hit flowing motions easier, and advanced button maneuvers as well as easier simultaneous button presses become possible. a Patroklos is an example of a character really aimed at stick players.
You can do everything on a pad that you can do on a stick- stick just makes some things easier.
Be aware that stick will not give you mad skills if they're not already in your head. It is just another flavor of execution.
A quirk about playing on pad is that double-taps are faster. Standard Sanwa sticks have this huge throw (distance from neutral to the end of the direction) so for beginner players (playing SC at least) it can seem like they've slowed down.
Learning curve can be somewhat steep. Hitting 1 (down-back) consistently was hard until I learned the EWGF trick from watching the Korean videos (pull in your middle and ring finger to hit 1). Really depends on your level of play, what you want to do, and what your philosophy/temperament is.
Hori and Madcatz have SCV branded sticks out now, with Sanwa parts (what is normally recommended). Though, there are other manufacturers that make sticks (eightarc, eTokki, etc....)
Using parts other than Sanwa (i.e. really off-brand parts) is not recommended. Sanwa buttons have a very fast response (they can activate if you blow on them- somewhat true, and somewhat an exaggeration, but you understand).
Most of the subtleties are lost on me, since I mash for most of my moves. I'm not terribly efficient with my hands.
If you have the money, or if you're curious (and you're not prone to feeling like you've been "ripped off") go ahead and try a stick. Using stick has actually taught me how to use pad better.
